Translocation and conservation of organic nitrogen within the coral-zooxanthella symbiotic system of Acropora pulchra, as demonstrated by dual isotope-labeling techniques

Carbon (C) and nitrogen (N) metabolism of the hermatypic coral Acropora pulchra and its symbiotic algae (zooxanthellae) was investigated using 13C and 15N isotope tracers. A. pulchra was incubated in seawater containing 13C-labeled bicarbonate and 15N-labeled nitrate (NO3−) for 24 h (pulse period), and subsequently 13C and 15N isotopic ratios of the host coral and the zooxanthellae were followed in 13C- and 15N-free seawater for 2 weeks (chase period). Under our experimental condition of NO3− (12 μM), C and N were absorbed by the coral–algal symbiotic system with the C:N ratio of 23 during the pulse period. Taking account of concentration dependence of NO3− uptake rates determined by a separate experiment, C:N uptake ratios under supposed in situ NO3− conditions (< 1.0 μM) would be > 3.0 times higher, if the photosynthetic rate did not change. During the pulse period, more than half of the absorbed 13C and 15N appeared in the host fraction in organic forms. 13C:15N ratio at the end of the pulse period was similar between the host and the algal fraction, suggesting that algal photosynthetic products were translocated to the host. It is also implied that C:N ratios of the translocated products change depending on N availability for the zooxanthellae. During the chase period, atom % excess (APE) 15N of the zooxanthellae constantly declined, while that of the host slightly increased. Consequently, APE 15N of the both fractions appeared to approach a common steady state value, suggesting that 15N was recycled within the coral–algal symbiotic system. As for C, > 86% of C photosynthetically fixed by the zooxanthellae accumulated in the host at the end of the pulse period, and had a turnover time of ca. 20 days for the host C pool during the following chase period. C:N ratios of organic matter newly synthesized with NO3− exponentially declined and converged into 5.7 and 4.5 for the host and the zooxanthellae, respectively. This suggests that organic compounds of high C:N ratios such as lipids and carbohydrates were selectively consumed more rapidly than those of low C:N ratios such as proteins and nucleic acids.